You Do Need power plug adapter while traveling from China to Vietnam.

Here is Why You Need An Adapter

The outlet and plug type used in Vietnam are A, B and C and in China are A, C and I. This is how they look like.

China

In China, outlets of type A, C and I are in use.

Type A

Type C

Type I

Vietnam

In Vietnam, outlets of type A, B and C are in use.

Type A

Type B

Type C